This invention relates to faders for radio signals.
A radio signal is said to be fading when the signal amplitude is observed to fluctuate at the receiver. In land-mobile radio, which uses frequencies that are usually restricted to line-of-site communication, fading is normally a result of the constructive or destructive interference which occurs among a number of different propagation paths. In the case of a fixed transmitter and a fixed receiver, fading results from the reflection of radio signals from moving objects, which causes the received sum to achieve values that vary. Similarly, in the case where a transmitter, a receiver, or both, are in motion, the effect of the relative motion between them may also produce a significant fluctuation in the received signal level. If the fluctuant signal reaches low values, the signal information is lost.
It is well known that the statistical distribution of fading is often that of a phasor having a phase angle that is distributed uniformly and having an amplitude that follows the Rayleigh distribution. Fading described by these statistics is called Rayleigh fading. An important measure of the performance of a radio receiver is its ability to handle a signal that has been subjected to Rayleigh fading. For this reason, Rayleigh faders have been designed and built as test equipment. In one form or another these faders split a signal into both in-phase and quadrature components, adjust the amplitudes of each of the quadrature components at a random rate, and recombine the adjusted components into a single component that is a Rayleigh-faded signal.
All of the faders that are known to date have been built for use in fading FM signals. An FM signal maintains a constant amplitude at the input to a fader. Faders currently in use are therefore designed to operate using an input signal that does not vary in amplitude. Such faders are not useful without modification in fading single-sideband or other AM signals. A single-sideband signal exists only when it is being modulated. When there is no modulation, there is no signal. Any fader that is to be used in fading single-sideband or AM must therefore work successfully with an input signal that may vary widely in amplitude.